1. Field of the Invention
The present invention relates to a duplexer for use in a communication device or other suitable apparatus, the filter including a piezoelectric thin-film filter having a piezoelectric thin-film resonator or a surface acoustic wave filter having a surface acoustic wave resonator, and a composite module having at least one filter and a mounting member.
2. Description of the Related Art
Recently, surface acoustic wave filters using surface acoustic waves and piezoelectric thin-film filters using bulk acoustic waves have been developed.
Moreover, duplexers having the above-described surface acoustic wave filters and piezoelectric thin-film filters are disclosed in Japanese Unexamined Patent Application Publication No. 2001-24476 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 9-181567 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2002-198774 (Patent Document 3).
Patent Document 2 discloses a structure in which a filter contained in a package is mounted on a printed circuit board, and the filter is covered in a metallic case. In this structure, the metallic case is connected to the ground (GND) of the printed circuit board, so that the case functions as a shield.
Patent Document 3 discloses a structure in which a filter contained in a package is mounted on a printed circuit board, and the filter is covered in a metallic case, the metallic case being connected to the ground (GND) of a printed circuit board so as to function as a shield. An insulating material layer is provided between the package and the metallic case, so that the electrical connection between the package and the metallic case is prevented. This structure prevents changes in parasitic inductance, which may be caused, e.g., when the case is distorted, resulting in the partial connection between the package and the case. Thus, the filter characteristic is prevented from being deteriorated.
However, according to the above-described structure, the filter contained in the package is grounded by its connection to GND of the printed circuit board via a land in the package, a through-hole in the package, a GND terminal of the package, and a through-hole of the printed circuit board. Therefore, it is difficult to provide a wide area for the GND. Moreover, the through-hole is narrow. Therefore, the electromagnetic field interference between the filters and that between the filters and a matching circuit element are insufficiently prevented. Thus, problems occur in that satisfactory attenuation and insertion loss can not be achieved.